A guniting nozzle of this type is known more especially from the document FR No. 2 419 484. More generally, guniting techniques have been the subject of numerous studies and patents: reference may for example be made to documents FR No. 2 168 916 and FR No. 2 066 355.
It is known that one of the major problems related to these techniques is that of obtaining a maximum rise of the temperature of the refractory powder before this latter reaches the wall to be repaired, so as to obtain the formation of a dense and homogeneous layer of product. If a sufficiently high temperature is not obtainable, products have to be used which are not very refractory and which are not suitable for all applications.
Pre-heating of the surface to be repaired does not provide an efficient solution to the problem.
The incorporation in the refractory powder of a fuel element such as carbon has not proved to be very favorable because of the increase of the porosity of the product projected due to the incomplete combustion of the carbon. Mixing with the powder a metal adapted to burn in oxygen with release of heat leads to a costly technique (for example in the case of magnesium), which is sometimes incompatible with the material to be deposited (for example, aluminum incompatible with MgO), which can be undesirable (for example, in the case of iron which lowers the refractoriness), and finally which can be dangerous (because of the risks of combustion of the metal in the transporting oxygen).
The nozzle described in document FR No. 2 419 484 in which the oxygen from a central duct is mixed, at the outlet of the nozzle, with natural gas conveying the refractory powder through an annular duct has the advantage of simplicity. However, the efficiency of the nozzle remains low (high power for a low powder temperature).